Mobile location estimation in a wireless communication system

ABSTRACT

Methods and apparatus for estimating mobile station location in a wireless communication system. At initiation of a call or a page response, a mobile station of the system sends an access request signal to a primary base station. The primary base station responds with an access acknowledgment which may be intentionally delayed such that the mobile station increases its transmit power level. The primary base station then transmits a channel assignment message to the mobile station. The mobile station responds by transmitting a location signal in the form of a known user-specific traffic preamble at the higher transmit power level. The primary base station intentionally delays sending an acknowledgment of the preamble, such that the mobile station transmits the preamble for a longer period of time than it would otherwise. This additional transmission time allows the preamble to be detected accurately in the primary base station as well as in a number of other surrounding base stations in a manner suitable for generating path delay measurements. A primary location processor associated with the primary base station uses the path delay measurements from the primary and surrounding base stations to generate an estimate of mobile location. The mobile location estimation may be provided during an on-going call by the primary base station sending a fake handoff message to the mobile station directing it to handoff to the same base station and traffic channel but with a different power level and preamble length, or by the primary base station sending a predetermined location probe message directing the mobile station to transmit the traffic preamble with a desired power level and preamble length.

This application is a continuation of pending application Ser. No.09/724,238, filed Nov. 28, 2000, now U.S. Pat. No. 6,785,550 which is acontinuation of U.S. application Ser. No. 08/775,329, filed Dec. 31,1996 now issued as U.S. Pat. No. 6,163,696.

FIELD OF THE INVENTION

The present invention relates generally to wireless communicationsystems and more particularly to techniques for estimating mobilestation location in a wireless code division multiple access (CDMA)system.

BACKGROUND OF THE INVENTION

Demand for wireless communication services, such as mobile telephones incellular and Personal Communications Service (PCS) systems, iscontinually increasing. An important issue in wireless communicationsystems involves the estimation of mobile station location. For example,the Federal Communications Commission (FCC) has requested that allcellular and PCS systems eventually include emergency 911 callerlocation capabilities similar to those provided in wired systems. Asreported in Radio Communications Report, Vol. 15, No. 51, Dec. 16, 1996,the FCC has required that Phase I of a wireless emergency 911 (E-911)system providing a 911 agent with caller number and cell site locationmust be completed by Apr. 1, 1998, while Phase II of the E-911 systemproviding caller latitude and longitude within a radius of no more than125 meters in at least 67% of all cases must be completed by Oct. 1,2001. A number of other services requiring mobile location estimationare also being considered, including routing guidance services, fleetmanagement and local commercial services. A wireless system which isable to determine the position of a given mobile station in an efficientmanner could thus provide an enhanced level of service to the user,while meeting the above-noted FCC requirements and also generatingadditional revenue for the service provider. It would also be veryadvantageous if mobile location estimation could be incorporated into agiven system without the need for any significant change to thestandards on which the system is based.

In order to estimate mobile location with an acceptable degree ofaccuracy, either the mobile station needs to be able to detect signalsfrom at least three surrounding base stations, or at least threesurrounding base stations need to be able to detect a signal from themobile station. The resulting signal propagation delay information canthen be processed in a conventional manner using triangularrelationships to derive an estimate of mobile location. Two importantaspects of mobile location estimation thus involve the manner in whichthe signals are detected in the mobile station or base stations, and theaccuracy of the propagation delay measurements required between themobile station and base stations. An exemplary prior art mobileestimation technique is described in M. Wylie et al., “The Non-Line ofSight Problem in Mobile Location Estimation,” ICUPC '95, Boston, Mass.,1995, which is incorporated by reference herein.

In the case of a code division multiple access (CDMA) systems such asthose based on the IS-95 standard, the implementation of a mobilelocation estimation capability presents a number of problems. AlthoughCDMA systems spread signals over a wider frequency spectrum thannarrowband systems such as TDMA, GSM and analog FM, and are thereforebetter able to resolve path delay ambiguity, the detection of signalsfrom surrounding base stations by the mobile or detection of mobilesignals by the surrounding base stations presents a greater challenge inCDMA systems than in narrowband systems. A basic principle of CDMAsystems involves the use of power control to solve near-field problemsand to control interference such that a capacity advantage can beachieved. Therefore, when the mobile station is not in a handoff zone,that is, when the mobile station is close to a base station, the signalstrength from the surrounding base stations is very weak. Similarly, thepower transmitted from the mobile is purposely made very small in orderto prevent interference. This means that in order to achieve thecapability of detecting signals from surrounding base stations in themobile station or detecting a signal from the mobile station insurrounding base stations, an excessively large signal-to-noise gain maybe required at the corresponding receivers. Application of conventionalmobile location techniques to CDMA systems may therefore requirealteration of basic system parameters, thereby increasing the cost andcomplexity of the system and possibly degrading system performance interms of interference. An exemplary CDMA mobile estimation technique isdescribed in J. Caffery et al., “Radio Location in Urban CDMAMicrocells,” Proceedings of PIMRC '95, pp. 858-862, IEEE, 1995, which isincorporated by reference herein. There are a number of problems withthis prior art technique and other similar techniques. For example, suchtechniques typically utilize either coarse timing acquisition or afiner-acquisition delay lock loop to obtain path delay information. Asnoted above, it is difficult to detect a mobile station signal at thesurrounding base stations unless an excessively large signal-to-noisegain is obtained, and the coarse timing and delay lock loop techniqueshave failed to solve this problem. In addition, the use of a delay lockloop usually requires the surrounding base stations to detect the mobilestation signal either continuously or for a very long detection period,thereby wasting system resources and significantly increasing systemcomplexity. Moreover, it is generally not feasible to utilize a delaylock loop in conjunction with an increased mobile station power level toassist acquisition because the resulting interference wouldsubstantially reduce system capacity.

As is apparent from the above, a need exists for an improved techniquefor estimating mobile station location in a wireless communicationsystem, and which can be implemented in a CDMA system without requiringany significant alteration to system operating and performancestandards.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for determiningmobile location in a wireless communication system. The invention allowsa mobile station to transmit a location signal which can be detected andprocessed in a primary base station and a number of surrounding basestations in order to generate accurate path delay measurements. Theprimary base station controls characteristics of the location signalsuch as power level and transmission duration in order to facilitatedetection by surrounding base stations while minimizing the interferencecreated by the location signal within the system.

In accordance with one aspect of the present invention, a mobilelocation estimation technique suitable for use at initiation of a callor page response is provided. A mobile station transmits an accessrequest signal and waits for an acknowledgment from a base station. Agiven base station receiving the access request signal is designated asa primary base station and has a primary location processor associatedtherewith. The primary location processor notifies a number ofsurrounding base stations, typically two other base stations, to preparefor detection of a location signal from the mobile station. The primarybase station then transmits an access acknowledgment to the mobilestation. The primary base station may intentionally delay thetransmission of this access acknowledgment such that the mobile stationincreases its transmit power level. The primary base station then sendsa channel assignment message to the mobile station. The mobile stationresponds to receipt of the channel assignment message by transmitting alocation signal at the increased transmit power level. The locationsignal may be a traffic preamble corresponding to a known set of dataspread by a pseudorandom noise (PN) sequence specific to the mobilestation. The primary base station and surrounding base stations detectthe traffic preamble and utilize it to generate path delay measurementsindicative of the path delay between the mobile station and the basestations. The primary base station intentionally delays the transmissionof an acknowledgment of the traffic preamble, such that the mobilestation continues to transmit the preamble for a longer period of timethan it would otherwise. This extended transmission time allows the basestations to utilize a longer integration period in the detectionprocess, resulting in more accurate path delay measurements. The primarylocation processor associated with the primary base station receives theaggregate path delay information from the base stations involved in theposition estimation, and uses the path delay information to estimate thelocation of the mobile station. After the base stations have detectedthe traffic preamble, the primary base station sends an acknowledgmentto the mobile station indicating that the channel set up is complete,and the call or page response can then take place over the trafficchannel.

In accordance with another aspect of the invention, mobile estimationtechniques suitable for use in on-going calls are provided. The mobileestimation techniques may be triggered by a request for mobile locationservice being received in a primary base station which is processing anon-going call for a particular mobile station. The mobile locationservice request may be in the form of a signal generated at theexpiration of a periodic timer, or a command received from a locationservice agent. A primary location processor associated with the primarybase station then directs a number of surrounding base stations toprepare for detection of a location signal to be transmitted by themobile station. The location signal may be in the form of a trafficpreamble typically generated by the mobile station during set up of thetraffic channel for a conventional call.

A first technique suitable for use during on-going calls involves theprimary base station transmitting a fake handoff message to the mobileuser directing the mobile user to handoff to the same base station andtraffic channel, but at a different power level and with a differentpreamble length. This fake handoff message may be provided using, forexample, the extended handoff direction message (EHDM) of an IS-95 CDMAsystem. Upon receiving the fake handoff message, the mobile stationtransmits the traffic preamble at the power level and preamble lengthspecified in the message. The preamble is detected in the primary andsurrounding base stations to generate the path delay information in themanner previously described. The primary base station then sends a powerdown command directing the mobile station to reduce its transmit powerin order to limit system interference, followed by an acknowledgment ofthe traffic preamble to complete the fake handoff process. The delayinformation generated by the various base stations is supplied to theprimary location processor associated with the primary base station, andused to estimate the mobile station location. The mobile station stopstransmitting the traffic preamble after receipt of the acknowledgmentfrom the primary base station, and the on-going call can then continueover the traffic channel.

A second technique suitable for use during on-going calls involves theprimary base station transmitting a location probe message (LPM) to themobile user directing the mobile user to transmit a traffic preamble orother predetermined location signal at a desired power level andpreamble length. Upon receiving the LPM, the mobile station transmitsthe traffic preamble at the specified power level and preamble length.The mobile station then immediately returns to a conversation mode andreduces its transmit power level to a power level used before sendingout the traffic preamble. As in the previously-described technique, thetraffic preamble is detected in the primary and surrounding basestations to generate the path delay information. The delay informationgenerated by the various base stations is supplied to the primarylocation processor associated with the primary base station, and used toestimate the mobile station location. The on-going call continues in thetraffic channel after the mobile station completes its transmission ofthe traffic preamble.

These and other features and advantages of the present invention willbecome more apparent from the accompanying drawings and the followingdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary code division multiple access (CDMA) wirelesscommunication system in which the mobile location techniques of thepresent invention may be implemented.

FIGS. 2A and 2B show exemplary implementations of mobile locationestimation techniques of the present invention utilizing a base stationand mobile switching center (MSC) of the wireless system of FIG. 1.

FIG. 3 is a block diagram of an exemplary signal detector suitable foruse in the base station of FIGS. 2A and 2B.

FIG. 4 shows an exemplary correlator suitable for use in the signaldetector of FIG. 3.

FIG. 5 is flow diagram illustrating mobile location estimation at theorigination of a call or a page response in accordance with a firstexemplary embodiment of the invention.

FIGS. 6 and 7 are flow diagrams illustrating mobile location estimationduring an on-going call in accordance with second and third embodimentsof the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be illustrated below in conjunction with anexemplary IS-95 code division multiple access (CDMA) wirelesscommunication system. It should be understood, however, that theinvention is not limited to use with any particular type ofcommunication system, but is instead more generally applicable to anywireless system in which it is desirable to provide cost-effectivemobile location estimation within the existing system performanceparameters. For example, although the techniques are illustrated withreference to the IS-95 CDMA cellular and personal communications service(PCS) systems, it will be apparent to those skilled in the art that thetechniques are also applicable to other CDMA systems, as well as toother types of wideband and narrowband wireless systems. The term“primary base station” as used herein refers generally to a base stationcommunicating directly with a given mobile station, such as the basestation handling an on-going call for the mobile station. The terms“surrounding base stations” or “secondary base stations” are intended toinclude those base stations other than the primary base station whichare in proximity to a given mobile station at a particular time and areused to generate the additional path delay measurements used to estimatemobile position. The term “location signal” refers to a signal havingcertain known characteristics, which is generated by a mobile stationand detected by primary and secondary base stations to generateaggregate path delay information for estimating mobile position. A“traffic preamble” is one type of location signal which may betransmitted by a mobile station over a traffic channel when initiating acall, responding to a page or during an on-going call.

FIG. 1 shows an exemplary cellular or personal communication services(PCS) system 10. The system 10 is configured in accordance withTIA/EIA/IS-95A, “Mobile Station—Base Station Compatibility Standard forDual-Mode Wideband Spread Spectrum Cellular System,” June 1996, and ANSIJ-STD-008, “Personal Station—Base Station Compatibility Requirements for1.8 to 2.0 GHz Code Division Multiple Access (CDMA) PersonalCommunication Systems,” both of which are incorporated by referenceherein. The system 10 includes a mobile station (MS) 12 and a number ofbase stations BS1, BS2, BS3 and BS4. The base station BS1 of FIG. 1represents a primary base station, communicating with mobile station 12via the path designated by solid two-way arrow 24, while the surroundingbase stations BS2, BS3 and BS4 may serve as secondary base stationswhich detect a location signal transmitted via the paths indicated bydashed one-way arrows 26. The primary base station BS1 communicates withthe mobile station 12 using CDMA techniques described in the above-citedstandards documents. As the mobile station 12 moves within the system10, handoffs may occur such that base stations other than BS1 becomeprimary base stations for communicating with the mobile station 12. Thesystem 10 also includes first and second mobile switching centers (MSCs)14-1 and 14-2. A given MSC typically connects several BSs with a publicswitched telephone network (PSTN) 16. In this exemplary embodiment, MSC14-1 connects base stations BS1 and BS2 with the PSTN 16 and MSC 14-2connects base stations BS3 and BS4 with the PSTN 16. The system 10 alsoincludes a memory 18 having a number of registers including a homelocation register (HLR) 20 and a visitor location register (VLR) 22. TheHLR 20 and VLR 22 store user data and billing information for eachmobile station 12 of the system 10.

FIGS. 2A and 2B show two alternative implementations of a mobilelocation service in accordance with the invention, illustrated using theprimary base station BS1 and MSC 14-1 of FIG. 1. In the FIG. 2Aimplementation, the base station BS1 includes a signal detector 30 fordetecting a location signal transmitted by a particular mobile station12 for which a location estimate is to be provided. As will be describedin greater detail below, the location signal may be in the form of atraffic preamble generated by the mobile station 12 at initiation of acall, initiation of a page response or during an ongoing call inresponse to an appropriate message signal transmitted from primary basestation BS1. The signal detector 30 provides signal power and path delayinformation regarding the detected signal to a location processor 32.The location processor 32 controls the timing and other operationalparameters of signal detector 30. The location processor associated withthe primary base station is designated as a primary location processorwith the responsibility to estimate the mobile location based on theaggregate information received from location processors of at least twoother surrounding secondary base stations. As noted above, the primarybase station is generally the base station communicating directly withthe mobile station 12. The base station BS1 in FIG. 2A further includesa message handler 34 which provides conventional call processingfunctions. The message handler 34 handles all messages communicatedbetween the primary location processor and the location processors ofthe surrounding base stations. The base station BS1 communicates withits corresponding MSC 14-1 via the message handler 34 as shown. The basestation BS1 also includes a timer 36 coupled to the location processor32. The timer 36 serves to activate the mobile location serviceperiodically if periodic probing of mobile location is requested. Theperiodic activation can be varied depending on the requirements of aparticular application.

FIG. 2B shows an alternative implementation in which the locationprocessor 32 is located in the MSC 14-1 rather than in the base stationBS1 as in FIG. 2A. The base station BS1 in this embodiment includes thesignal detector 30 and message handler 34 as previously described. Theexemplary MSC 14-1 includes the location processor 32, the timer 36 anda call processor 38. The call processor 38 handles all messagescommunicated to or from the MSC including those relating to the locationprocessor 32. The operation of the location processor 32 and timer 36 inboth the FIG. 2A and FIG. 2B implementations will be described ingreater detail below in conjunction with the flow diagrams of FIGS. 5, 6and 7. It should be understood that although FIGS. 2A and 2B illustrateonly a single base station, the other base stations in the system willalso be configured in a similar manner.

FIG. 3 shows an exemplary signal detector 30 suitable for use within thebase station BS1 of FIG. 2A or FIG. 2B to estimate the signal power andpath delay of a signal received from the particular mobile station 12 tobe located. The signal detector 30 includes a number of correlators40-i, i=1, 2, 3, . . . N each of which receives the incoming locationsignal transmitted by the mobile station 12 and correlates the incomingsignal with a different path delay hypothesis τ_(i). Determining thenumber of different path delay hypotheses to be used in a particularembodiment generally involves a trade off between hardware cost andprocessing delay, with more correlators increasing the hardware cost butdecreasing the processing delay. For example, certain embodiments mayreduce hardware costs by performing sequential detection using arelatively small number of correlators, with a corresponding increase inprocessing delay. An exemplary embodiment may utilize about 10correlators to detect about 80 different path hypotheses using a timingresolution of about 406 nanoseconds. Of course, numerous otherimplementations having different arrangements of correlators, pathhypotheses and timing resolutions may also be used. The detector 30further includes a threshold and select circuit 42 which selects theoutput of the correlator 40-i having the earliest arrival with a signalpower greater than a detection threshold in order to determine the bestpath delay hypothesis. As previously noted, the signal power and pathdelay information generated in a given base station are supplied to thelocation processor 32 of the primary base station either directly asshown in FIG. 2A or indirectly via the message handler 34 and callprocessor 38 as shown in FIG. 2B.

FIG. 4 shows an exemplary correlator 40-i suitable for use in the signaldetector 30 of FIG. 3. The incoming location signal received from theparticular mobile station is applied to mixers 52, 54 which are part ofcorresponding in-phase (I) and quadrature (Q) detection channels,respectively. The I-channel mixer 52 receives a local oscillator (LO)signal directly from an LO 56, while the Q-channel mixer 54 receives a90° phase-shifted version of the LO signal from a phase shifter 58. Theoutputs of the mixers 52, 54 are filtered in respective low pass filters60, 62 and applied to mixers 64, 66. The mixers 64, 66 multiply theuser-specific pseudo-random noise (PN) long code with the receivedsignal outputs of the respective filters 60, 62. The long code has atiming epoch selected in accordance with the path delay hypothesis τ_(i)of the correlator 40-i. On the hypothesis that the path delay betweenthe mobile station and a given base station is τ_(i), the base stationshifts its user-specific PN long code, as well as I-channel andQ-channel PN sequences used to obtain diversity in the system, by anappropriate amount relative to the system time. For example, the long PNcode and the I-channel and Q-channel sequences may be shifted by anamount equal to about 2τ_(i) for detecting round trip path delayrelative to a system time corresponding to a zero path delay, and theincoming signal is then decoded/de-spread using the shifted sequences. Ahigh output power after the de-spreading operation associated with thehypothesis τ_(i) indicates the existence of such a path delay componentin the signal propagation path between the mobile and the base station.

The outputs of the mixers 64, 66 are applied to I-channel multipliers68, 70 and Q-channel multipliers 72, 74 as shown. The I-channelmultipliers 68, 70 multiply the output of respective mixers 64, 66 usingthe I-channel PN sequence, and the Q-channel multipliers multiply theoutput of respective mixers 64, 66 using the Q-channel PN sequence. Asnoted above, the I-channel and Q-channel PN sequences also have a timingepoch corresponding to the path delay hypothesis τ_(i) of the correlator40-i. The outputs of multipliers 68, 70, 72 and 74 are integrated incorresponding integrators 76, 78, 80 and 82. The output of integrators76 and 82 are summed in a summing device 84 to produce a signal S_(A),and the outputs of integrators 80 and 78 are summed in a summing device86 to produce a signal S_(B). If the timing epoch associated with pathdelay τ_(i) of correlator 40-i is correct, the signals S_(A) and S_(B)will be of the form:S _(A) =x cos θ+noiseS _(B) =x sin θ+noisewhere x is the desired signal amplitude and θ is an unknown phaserotation during the integration period of integrators 76, 78, 80 and 82.A phase lock loop 88 is used to estimate the phase rotation θ. The phaserotation estimate from phase lock loop 88 is applied to circuit elements90, 92 to generate respective cosine and sine values of the phaserotation estimate. These values are used in multipliers 91 and 93 tode-rotate the respective signals S_(A) and S_(B) such that an estimateof x is obtained at the output of a summing device 94. Exemplary phaselock loops suitable for use estimating the phase rotation θ incorrelator 40-i are described in greater detail in W. C. Lindsey,“Synchronization Systems in Communications,” Prentice-Hall, 1972,Englewood Cliffs, N.J.

The correlator 40-i further includes an accumulator 96 which is used toprovide an effective extension of the integration period such that ahigher signal-to-noise ratio can be achieved. Within a given correlator,the extended integration may be implemented as follows. If a netintegration period T is needed based on the required signal-to-noiseratio and the relative power transmitted from the mobile station, thepath delay estimation error is reduced by dividing T into a number N ofmutually disjoint periods T₁, T₂, . . . T_(N) where T₁+T₂+ . . .T_(N)=T. It should be noted that a given integration period T_(i) andthe above-described de-rotate/estimation process should be configuredsuch that the phase rotation within each integration period T_(i) isapproximately constant. Suitable integration periods T_(i) for anexemplary system may be in the range of about 1 to 5 milliseconds, butmay vary in accordance with application-specific factors such as vehiclespeed.

The mobile station 12 of system 10 in accordance with present inventiontransmits a location signal which may include a known sequence having adesired power level such that a signal detector 30 in a primary basestation and at least two other surrounding base stations can detect thesignal and estimate the path delay using correlators 40-i as previouslydescribed. A location processor 32 associated with the primary basestation can then use the aggregate path delay information to estimatemobile location. A number of exemplary techniques for operating themobile station and base stations of system 10 to provide the desiredsignal transmission and corresponding path delay measurements will bedescribed in conjunction with the flow diagrams of FIGS. 5, 6 and 7.

FIG. 5 is a flow diagram of a mobile location estimation process inaccordance with a first embodiment of the invention. This embodiment isparticularly well-suited for use in estimating mobile location at theinitiation of a call or a paging response. The above-cited IS-95standard requires the mobile station to transmit a dial-up trafficpreamble after it receives a traffic channel assignment message from abase station. This traffic preamble is a known sequence in the form ofall zeros which is subsequently spread by the user-specific PN sequence.The mobile station continuously transmits the traffic preamble until itreceives an appropriate response from the a base station or until theexpiration of a maximum timeout period which is usually about twoseconds. The primary base station intentionally delays the transmissionof a response to the traffic preamble such that the mobile stationtransmits the preamble for a longer period of time that it wouldotherwise, in order to provide an extended integration period. A numberof surrounding base stations can also detect the traffic preambletransmitted by the mobile station, and can use the above-noted extendedintegration period to obtain sufficient signal-to-noise ratio togenerate the necessary path delay information. Since the trafficpreamble is a known sequence, the probability of correct detection willbe substantially higher than that of an unknown data-modulated sequence.Also, the fact that the traffic preamble is spread by the user-specificPN sequence helps the base stations to identify the correct mobilestation signal from among those of other mobile stations.

The power level at which the traffic preamble is transmitted isdetermined in accordance with the invention as the transmit power levelof the mobile station when it receives an access acknowledgment from thebase station. In accordance with the above-cited IS-95 standard, themobile station uses a nominal initial transmit power level for an accessrequest signal to request services such as a call origination or pageresponse. If the mobile station does not receive an acknowledgment ofits access request within a particular time period, which may bespecified by a base station in a broadcast message, the mobile stationwill re-send the request but at a higher power level offset from theprevious power level. Like the initial transmit power level, the offsetmay be specified in a broadcast message from the base station. Thisprocess is repeated until the mobile station receives an acknowledgmentof its access request, and helps the mobile station get its accessrequest through to the base station at a proper power level. Afteracknowledging the access request, the base station transmits a channelassignment message to the mobile station. Upon receipt of the channelassignment message, the IS-95 standard requires the mobile station totransmit the traffic preamble at the same power level that it lasttransmitted before it received the access acknowledgment. Therefore, ifthe base station intentionally delays the transmission of the accessacknowledgment to the mobile station, the subsequent transmit powerlevel which the mobile station uses to send the dial-up traffic preamblewill be higher than it would be otherwise. The present inventionutilizes this technique to allow a primary base station to control thetransmit power of the mobile station during mobile location estimation.

These aspects of the invention are illustrated in steps 100 to 110 ofFIG. 5. Step 100 indicates that the mobile location estimation may beinitiated for a call origination or a page response. In step 102, themobile station sends out a conventional access request signal,indicating to nearby base stations that the mobile user is requestingaccess to the system, and waits for an acknowledgment. In step 104, oncea given base station receives a valid access request signal from themobile station, that base station becomes the primary base station. Atleast two other surrounding base stations will serve as secondary basestations in generating path delay information needed to estimate themobile location. The location processor 32 associated with the primarybase station, referred to as the primary location processor, notifies atleast two other surrounding base stations to prepare for detection of alocation signal in the form of a traffic preamble to be transmitted bythe mobile station. If necessary, the primary base station willintentionally delay the transmission of an acknowledgment signal to themobile station in order to force the mobile station to increase itssubsequent transmit power level for the traffic preamble. Thedetermination of whether to provide this intentional delay may be basedon the mobile station signal power levels detected at the primary basestation. If the intentional delay is provided, it may be on the order ofone or two access request signals, where the delay between accessrequest signals is specified in a broadcast message from the basestation, in order to provide a net transmit power level increase ofabout 6 to 10 dB. After acknowledging the access request, the primarybase station transmits a channel assignment message to the mobilestation. Step 106 indicates that the mobile station responds to receiptof the channel assignment message by transmitting the traffic preambleand waiting for an acknowledgment from the primary base station.

In step 108, the surrounding base stations each perform the signaldetection described in conjunction with FIGS. 3 and 4 above in order toestimate the relative path delay associated with the traffic preambletransmitted by the mobile station. The primary base station delays thetransmission of its traffic preamble acknowledgment to the mobilestation such that the surrounding base stations can have sufficient timeto perform the necessary signal detection. For example, this intentionaldelay may be selected in the range from about 0.2 seconds to about 1.2seconds in an exemplary embodiment. After the intentional delay, theprimary base station sends a traffic preamble acknowledgment signal tothe mobile station. The surrounding base stations then report therelative path delay information to the primary location processor 32associated with the primary base station via one or more of theabove-described MSCs. The primary location processor 32 uses theaggregate delay information from the primary and surrounding basestations to estimate the mobile station location. For example, theprimary location processor may utilize a conventional triangularizationtechnique with path data from three base stations which received themobile station traffic preamble to determine the mobile location. Suchtechniques are well-understood in the art and will therefore not bedescribed in detail herein. The primary location processor 32 then sendsthe mobile position estimate to the appropriate service agent or agentsfor further processing in accordance with the particular service. Forexample, the mobile position may be used to generate directions, mapdisplays or other audio and/or video information which can be suppliedto the mobile station in a suitable format. The traffic channel set upis then complete and a user conversation can take place over the trafficchannel in a conventional manner, as indicated in step 110.

FIG. 6 is a flow diagram of a mobile location estimation process inaccordance with a second embodiment of the invention. This embodiment isparticularly well-suited for use in estimating mobile location during anon-going call in which user conversation data is carried over thetraffic channel, and involves the use of a fake hard handoff during theon-going call. The primary base station directs the mobile station totransmit the traffic preamble at a desired power level using an theextended handoff direction message (EHDM) of the IS-95 CDMA system. TheEHDM was designed to direct the mobile station to perform a hard handoffbetween sectors and/or frequencies of a given base station or betweendifferent base stations. The present invention utilizes the EHDM in afake handoff, that is, a handoff to the same sector and frequencycurrently being used by the mobile, in order to direct the mobilestation to send out the desired traffic preamble at the preferred powerlevel such that surrounding bas stations are able to detect it. Itshould be noted that the EHDM of IS-95 permits a primary base station tospecify the length of the traffic preamble, such that a longerintegration time can be provided, as well as a power offset to the openloop power estimate. With the high speed reverse link power control ofthe IS-95 standard, the increased interference resulting from the highertransmitted power of the mobile stations can be reduced at a speed of 16dB/frame, where a frame is 20 milliseconds in duration. It should benoted that in applications in which only a small percentage of usersrequire the location services, individual mobile stations may bedirected to transmit the traffic preamble for the base stations todetect. If a high percentage of users require the location services,synchronized locate commands may be utilized to minimize the frame errorrate.

Step 120 of FIG. 6 indicates that an on-going call is in progress overthe traffic channel when the mobile location process is initiated. Thelocation process is initiated in step 122 by the expiration of theperiodic timer 36 of the primary base station or the corresponding MSC,or the receipt of a command from a location service agent. The primarylocation processor 32 associated with the primary base station servingthe on-going call then notifies the surrounding base stations to preparefor detection of the traffic preamble associated with the particularmobile station to be located. The primary base station sends out theEHDM to perform a fake hard handoff to the same base station, sametraffic channel and same frequency currently being used for the on-goingcall. In the EHDM, the primary base station directs the mobile stationto send out the traffic preamble with a desired power offset andpreamble length. For example, the EHDM may specify a power offset ofabout 5 to 12 dB, and a preamble length of about 2 to 7 frames. Ofcourse, other values could be used in alternative embodiments. In step126, the mobile station transmits the traffic preamble as directed bythe base station.

In step 128, the surrounding base stations perform the signal detectiondescribed in conjunction with FIGS. 3 and 4 above in order to estimatethe relative path delay of the received traffic preamble. After thesignal detection is completed, the primary base station sends a powerdown command to the mobile station in order to minimize interference,and then sends an acknowledgment to the mobile station to complete thehandoff process. The surrounding base stations report the relative pathdelay information to the primary location processor 32 associated withthe primary base station, and the primary location processor utilizestriangularization or another suitable technique to estimate the mobilelocation from the aggregate delay information. The primary locationprocessor then sends the mobile station location estimate to theappropriate location service agent or agents. The user conversation ofthe on-going call can then continue in the traffic channel, as shown instep 130.

FIG. 7 is a flow diagram of a mobile location estimation process inaccordance with a third embodiment of the invention. The above-describeduse of the EHDM to direct the mobile station to transmit the trafficpreamble has a number of disadvantages in certain applications. Forexample, the transmit power level for the preamble is indirectlyspecified and therefore may not be sufficiently accurate. In addition,the maximum power down speed after detection of the preamble in thenecessary base stations is about 16 dB/frame, which might cause someamount of additional interference in the system. The third embodimentillustrated in FIG. 7 overcomes these disadvantages of the secondembodiment by creating a new message in IS-95 or other similar standardin order to allow the base station to direct the mobile station totransmit a preamble or other known sequence at the desired startingepoch, with the desired length and at the desired power level. Thedesired power level may be specified as a power level offset to be addedto a closed loop power control result. This new message allowssurrounding base stations to detect the mobile signal and generate thepath delay information required to estimate mobile location. Asignificant advantage of employing this new location message is that themessage does not involve the reverse link power control, and can directthe mobile station to return to an original power level immediatelyafter transmitting the preamble at a higher power level. This provides amore effective reduction of the increased interference penaltyassociated with the higher transmit power level of the traffic preamble.

Step 140 of FIG. 7 indicates that an on-going call is in progress overthe traffic channel before the mobile location process is initiated. Thelocation process is initiated in step 142 by the expiration of theperiodic timer 36 of the primary base station or the corresponding MSC,or the receipt of a command from a location service agent. The primarylocation processor 32 associated with the primary base station servingthe on-going call then notifies the surrounding base stations to preparefor detection of the traffic preamble associated with the particularmobile station to be located. The primary base station sends out alocation probe message (LPM) to direct the mobile station to send out atraffic preamble at a desired power level and desired preamble length.The power levels and preamble length may be the same as those used forthe FIG. 6 process in a given application. In step 126, the mobilestation receives the LPM from the primary base station and responds bytransmitting the traffic preamble using the power level and preamblelength specified in the LPM. Immediately after sending out the trafficpreamble, the mobile station returns to the conversation mode andreduces the transmit power to the level used before sending thepreamble.

In step 148, the surrounding base stations perform the signal detectiondescribed in conjunction with FIGS. 3 and 4 above in order to estimatethe relative path delay of the received traffic preamble. After thesignal detection is completed, the surrounding base stations report therelative path delay information to the primary location processor 32associated with the primary base station, and the primary locationprocessor utilizes triangularization or another suitable technique toestimate the mobile location from the aggregate delay information. Theprimary location processor then sends the mobile station locationestimate to the appropriate location service agent or agents. The userconversation of the on-going call can then continue in the trafficchannel, as shown in step 150.

The present invention may utilize a number of different techniques foractivating the above-described mobile location services. One techniqueinvolves checking the number that the mobile user is dialing, andactivating the location service automatically if the dialed number is ina group of predetermined numbers for which the location service isprovided. For example, the location service may be activatedautomatically each time a user dials an emergency number such as 911 ora roadside assistance number. This type of activation could be providedusing message handler 34 of a given base station to provide anactivation indication to a corresponding location processor 32. Also,the activation of the location service may involve checking user data inthe HLR 20 and VLR 22 of the system memory 18 of FIG. 1 to determine ifa given user requires or is entitled to grade of service which includesthe mobile location estimation. In other embodiments, the system maycheck with a party in the communication link other than the mobile userto determine if that party has requested mobile location service. Theother party may be, for example, a governmental authority such as theFBI, or a private business entity owning a number of mobile stationsused by its employees. Such activation can also be performed using theHLR 20 and VLR 22 of the system memory 18.

The above-described embodiments of the invention are intended to beillustrative only. Numerous alternative embodiments may be devised bythose skilled in the art without departing from the scope of thefollowing claims.

1. A method of estimating mobile station location in a wirelesscommunication system in which a mobile station communicates with aprimary base station associated with a primary location processor, themethod comprising the steps of: transmitting from the primary basestation a message signal directing the mobile station to transmit alocation signal, wherein the message signal comprises a location probemessage signal directing the mobile station to transmit the locationsignal at a particular power level and for a particular duration, suchthat the mobile station returns its transmit power level to a powerlevel utilized prior to sending the location signal after the locationsignal has been sent at the power level and duration indicated in thelocation probe message signal; detecting the location signal in theprimary base station to generate a first path delay measurement;receiving additional path delay measurements generated based ondetection of the location signal in a plurality of secondary basestations; and utilizing the first and additional delay measurements toestimate the location of the mobile station.
 2. An apparatus forestimating mobile station location in a wireless communication system,the apparatus comprising: a primary base station for communicating withthe mobile station, the primary base station transmitting a messagesignal directing the mobile station to transmit location signal, theprimary base station detecting the location signal to generate a firstpath delay measurement, wherein the message signal comprises a locationprobe message signal directing the mobile station to transmit thelocation signal at a particular power level and for a particularduration, such that the mobile station returns its transmit power levelto a power level utilized prior to sending the location signal after thelocation signal has been sent at the power level and duration indicatedin the location probe message signal; and a primary location processorassociated with the primary base station, the primary location processorreceiving the first path delay measurement, receiving additional pathdelay measurements generated by a plurality of secondary base stationswhich have detected the location signal, and utilizing the first andadditional delay measurements to estimate the location of the mobilestation.
 3. An apparatus for estimating mobile station location in awireless communication system, the apparatus comprising: a mobilestation operative to receive a message signal from a primary basestation of the system, wherein the message signal comprises a locationprobe message signal directing the mobile station to transmit thelocation signal at a particular power level and for particular duration;wherein the location signal is detectable in the primary base stationand a plurality of secondary base stations for utilization in generatingpath delay measurements which can be processed to estimate the locationof the mobile station; wherein the mobile station returns its transmitpower level to a power level utilized prior to sending the locationsignal after the location signal has been sent at the power level andduration indicated in the location probe message signal.
 4. A method ofestimating mobile station location in a wireless communication system,the method comprising the steps of: receiving a message signal from aprimary base station of the system, wherein the message signal comprisesa location probe message signal directing the mobile station to transmitthe location signal at a particular power level and for a particularduration; and transmitting the location signal at the power level andduration indicated in the location probe message signal; wherein thelocation signal is detectable in the primary base station and aplurality of secondary base stations for utilization in generating pathdelay measurements which can be processed to estimate the location ofthe mobile station; wherein the mobile station returns its transmitpower level to a power level utilized prior to sending the locationsignal after the location signal has been sent at the power level andduration indicated in the location probe message signal.